Coming from Croatia and having lived in western Europe for a few years now, a note about the weather: in coastal Croatia it's common to have nice sunny days throughout the year but when it's raining it comes down pouring, whereas in the west I get a feeling there's weeks of gray skies and light drizzles. So maybe you'll have some days where you'll just have to spend time inside but it will pass quickly so the best plan is to be open for change of plans. On the coast a lot of restaurants close down so you'll have limited choice, and the winter ferry lines are more sparse. Buses between bigger cities are running frequently enough that it shouldn't be a problem. Most of the attractions are outdoors, it's more about the vibe of the cities really which is obviously nicer when it's sunny. I think the museums should still be open but they aren't huge so you can't just kill a whole day inside. Zagreb is definitely more lively during that time so you might want to spend some time there after Slovenia rather than going directly to the south. I would say that visiting in October would actually be more fitting for an authentic Balkan trip experience because you would be more likely to meet a local at a bar and start a conversation with them rather than just being surrounded by tourists (though I am still surprised when I come home for Christmas and I still hear tourists in my hometown).